Intermolecular Forces

Introduction

• Definition of Intermolecular Forces

  • Forces that act between separate molecules.

  • Relevant for molecular compounds, not ionic or network covalent compounds.

Types of Intermolecular Forces

• Hydrogen Bonding

  • Strongest type of intermolecular force.

  • Occurs when hydrogen is bonded to highly electronegative atoms: fluorine, oxygen, or nitrogen.

  • Example: Water (H₂O) has strong hydrogen bonds due to its polar nature.

• Dipole-Dipole Forces

  • Occur between polar molecules with permanent dipoles.

  • Example: HCl has a polar bond due to the difference in electronegativity between hydrogen and chlorine.

  • The strength of dipole-dipole forces increases with the polarity of the molecules.

• London Dispersion Forces

  • Present in all molecules, including nonpolar ones.

  • Result from temporary dipoles created by the movement of electrons.

  • Strength depends on molecular size and surface area; larger molecules have stronger London dispersion forces.

• Ion-Dipole Forces

  • Occur between a polar molecule and an ion.

  • Generally stronger than hydrogen bonding.

  • Example: Dissolving sodium chloride (NaCl) in water involves ion-dipole interactions.

Comparison of Intermolecular Forces

• Ranking of Forces

  • Hydrogen bonding > Dipole-dipole forces > London dispersion forces (for similar-sized molecules).

  • Ion-dipole forces can be stronger than hydrogen bonds but are not classified as intermolecular forces.

Effects on Bulk Properties

• Boiling Point

  • Transition from liquid to gas requires breaking intermolecular forces.

  • Stronger intermolecular forces result in higher boiling points.

• Melting Point, Viscosity, Surface Tension

  • Similar relationships exist; stronger intermolecular forces lead to higher melting points, increased viscosity, and greater surface tension.

Conclusion

• Understanding intermolecular forces is crucial for predicting the physical properties of substances.

• The strength and type of intermolecular forces significantly influence boiling points, melting points, and other